It has been recognized that the productivity of natural gas wells deteriorates over time as a result of a loss in reservoir pressure. The loss in reservoir pressure may lead to flooding as a consequence of the ingress of water and hydrocarbon (condensate) from the surrounding formation.
Symptoms of flooding include a drop in the gas velocity or erratic flow caused by liquid aerosols condensing and accumulating to form ‘slugs’ of fluid. If the liquid is not removed from the well, the fluid level and hydrostatic pressure will increase over a period of time until it reaches a point where it will effectively block the flow of gas to the surface, and from the reservoir into the well.
Well productivity can be improved or maintained by the implementation of artificial lift programmes. These programmes include mechanical methods such as plunger and hydraulic lift, or chemical treatments, in particular foam assisted lift. Surfactants are an integral component of the chemical formulations used in foam assisted lift to unload fluids. Foam assisted lift is cost effective, as well as offering both environmental and performance benefits. Methods for liquid unloading by foam assisted lift are well known in the art.
Foam assisted lift, as its name infers, uses surfactants to create a ‘stable’ matrix to trap the accumulated fluid (water and hydrocarbons) within the foam's structure. Once formed the foam may then be removed from the reservoir.
The foam is created by shearing forces present in the liquid and gas bubbles are stabilised by the adsorption of surfactant molecules at the gas/liquid interface. The surfactant stabilises the interface by establishing a barrier to coalescence as a result of repulsive steric and electrostatic head group interactions.
Surfactant adsorption also modifies the properties of the interface, e.g. reducing the surface tension. In so doing, it becomes easier to create new interfacial area and the dispersion of gas bubbles is stabilised by further adsorption of surfactant molecules from the bulk liquid. The bubbles continue to multiply and the liquid is incorporated within the film or lamellae separating the bubbles. Lifting the liquid to the surface is permitted when the density of the liquid falls to a value where the threshold flow for lift is achieved, i.e. the critical velocity (Turner Model). In practice, the well is shut for a period of time after the surfactant composition is injected to allow the gas pressure to build and to lower the density of the trapped fluids.
Surfactant formulations are available in both liquid and solid formats. Surfactants from the main chemistry groups have been used in these formulations and include anionic, cationic, amphoteric and non-ionic amphiphiles.
There is, however, an ongoing need for improved surfactant compositions. The present invention addresses this and other problems with the prior art.